10 YEARS OF EXPERIENCES OF AN MBBR PROCESS FOR POST-DENITRIFICATION

Problems with eutrophication and algae blooms initiated the need for enhanced nutrient removal at European wastewater treatment plants in the end of the eighties. When upgrading the two WWTPs in Malmö, site specific upgrading concepts had to be considered because of the existing plant structures. As phosphorus removal already was implemented, the upgrading was focused on enhanced nitrogen removal. The key element in the upgrading procedure was the retrofit with an appropriate post-denitrification process. A newly developed concept with a moving bed biological reactor process was found to be cost-effective. Pilot plant tests with an MBBR process for post-denitrification were performed for demonstration purpose and to get some ideas for proper design criteria. The later experiences from the full-scale plant operation showed good agreement with the selected design criteria from the pilot plant data. The future effluent standards could be met by the upgrading of the plants. The pilot plant tests as well as the experiences from full-scale operation pointed out the need for a control system for external carbon dosage. The control action was based on the instantaneous nitrate load calculated from on-line measurements of nitrate and wastewater flow. The need for an active control of the phosphorus concentration in the MBBR process was also stressed by the experiences.